Do not use the word "the" for the domain name, I do not know why, but you can look it up on google. I prefer a named domain carforums.com
CarForums.com of course, The only time you can use "the" in my opinion is when you getting a name that possibly oneday be some company name and they can buy it from you. expl: TheSignFactory.com, TheScooterStore.com etc
carforums.com is better.... seems like some of the last good .com domain names available.. so go 4 it rather than thecarforum
thecarforum.com is better than carforums.com 100% because first one is actually look like a forum but on the other hand 2nd one looks like a directory of car forums but carforum.com is better than thecarforum.com
CarForums.com gets a vote from me as well. Of course, it's not CarForum.com, but still better than TheCarForum.com.
carforums.com - I've had a similar generic+forums site built from the ground up, and just seemed like an intuitive name. solid name for sure